The present invention relates to method and apparatus for printing indicia, such as alpha-numeric characters, on an exterior three-dimensional surface of a continuous, elongate, flexible member, such as an electrically insulated wire that is to be marked for identification.
In the manufacture and servicing of multi-wire electrical systems, it is useful and many times necessary to mark the wires with identifying alpha-numeric codes or other indicia. For example, the electrical control systems for a commercial aircraft utilize thousands of control wires which are strung throughout the aircraft frame. Usually, the wires are grouped to form a wire harness wherein the wires are measured and cut to predetermined length, and routed to fit between the plugs, connectors and other terminals pre-established for each harness. The harness is thereafter installed in the aircraft.
In the manufacture and installation of these wire harnesses and subsequently during servicing the aircraft's electrical system, it is necessary to be able to identify and distinguish each wire from the numerous adjacent wires in the same or similar harness. Because of the large number of wires, mere color coding of the insulating sheaths is not satisfactory, and it has become necessary to resort to lengthy, multiple character alpha-numeric codes to uniquely identify each wire and group of wires.
Heretofore, these alpha-numeric identification codes have been either stamped directly onto the insulating sheaths of the wires or applied indirectly by placing printed tape or sleeve tags on the wires at spaced intervals therealong. Of these processes, hot-stamp marking and cold ink type wheel marking techniques have been preferred. In hot-stamp marking, each of the wires to be marked are transported into a printing position, stopped and momentarily held stationary while a heated alpha-numeric type face is pressed into engagement with the exterior insulating sheath of the wire. The type is either pre-inked or an ink ribbon is interposed between the type and the wire sheath, causing the ink on the ribbon to be transferred to the sheath in a process which has sometimes been called "branding" of the wire. After each "branding" or printing operation, the wire is advanced by a predetermined longitudinal interval, whereupon the operation is repeated. This technique involves a number of disadvantages. The start-stop, intermittent advancement of the wire, significantly limits the overall operating speed of the process. Furthermore, substantial labor cost is required in operating the machines and in changing the alpha-numeric code type, each time a new batch of wire is processed.
Moreover, it has been found that the heated, printing type, when pressed into contact with the wire insulation pursuant to the "branding" operation, causes significant deformation of the insulation, sometimes damaging it beyond acceptable specifications. In other cases, the "branding" results in an intolerable change in the effective dielectric of the insulation. This degradation of the insulation and the change in the dielectric thereof, is particularly troublesome in the case of wires having thin wall insulation. The thinness of such wire sheaths is incapable of accommodating any significant penetration of the heated type, thus limiting the application of this type of marking process.
In cold ink, type wheel printing, the type is disposed about an outer circumferential rim of the wheel and is rotated with such rim in engagement with the wire to transfer the printing ink thereto. Thermal curing of the transferred ink is thereafter required. Practical limits on the wheel rotation speed, inflexibility of the wheel code fixed by the type, and labor costs in changing type wheels and other limitations have been encountered in utilizing this technique.
Because hot-stamp, type wheel and other automatic contact printing processes require a relatively uniform printing surface, they are incapable of satisfactorily printing code numbers on wire surfaces that are irregular, such as exhibited by twisted or straight stranded, braided, and coax wires. With wires of this type, the exterior surface is too irregular to be printed by type and it is necessary to resort to slower, more costly techniques, such as manual application of preprinted tags.
Since contact printing systems, such as the hot-stamp technique, typically involve the use of alpha-numeric type, there ia usually a certain maximum number of characters, e.g. 18 to 25, available for forming a particular identification code. This, of course, limits flexibility in the amount and kind of the wire identification data that can be printed onto the wire. These foregoing disadvantages, present restrictions on the efficiency and reliability of existing marking systems.